This invention relates to fluid pressure operated actuators of the form having an internal spring means for biasing the actuator towards a predetermined state of contraction or extension. More particularly, the invention is an actuator of this general kind having safety means for preventing sudden release of the spring force upon disassembly of the device.
Fluid pressure operated actuators include a cylindrical housing from which a movable piston rod extends in an axial direction to transmit force to some external load and are further provided with port means for admitting pressurized fluid into the cylindrical housing to force piston rod movement when the device is to be actuated. Such actuators are often one way devices in which fluid pressure acts to move the piston rod in only one direction while an internal spring returns the piston in the opposite direction when the fluid pressure is released.
While the term actuator is herein used to designate devices of the kind discussed above, essentially similar mechanisms are variously known in the art as fluid cylinders, hydraulic jacks, linear fluid motors or by still other names and the term actuator as used herein should be understood to refer to all such devices which have the basic structure discussed above.
In many instances, the spring induced movement of the piston rod is relied upon for exerting a desired force on a load while the opposite movement produced by fluid pressure is simply a means of relieving this force when that is necessary. In these usages of fluid actuators, the necessary spring force may be very large and thus the spring itself is strong and capable of storing a very sizable amount of energy within the actuator. Considering one typical example, pipe laying vehicles have booms for supporting a load through a cable which is wound upon a drum that is controllable by a motor. For safety reasons, the drum is provided with a brake band controlled by a one-way fluid actuator of the form discussed above When the drum is to be turned, the brake is temporarily released by applying pressurized fluid to the actuator. In the absence of such pressurized fluid, an internal spring contracts the actuator to brake the cable drum. In such usage, the spring force present in the unpressurized actuator may be as high as 800 pounds.
In usages such as that described above, the energy stored in the spring within a fluid actuator is of such a magnitude that a very serious hazard has existed when the actuator is to be disassembled for repair, replacement of worn components or for other reasons. Upon disengagement of the end member of the actuator, the spring may expand violently and may propel the end cap and other internal components outwardly with substantial force. This can result in damage to the components and creates a serious risk of injury to nearby personnel.